1. Field of the Invention
The invention relates to a method for producing a functionally gradient material and, more particularly, to a method for producing a functionally gradient material using a preceramic polymer binder, as well as to materials so produced.
2. Description of the Prior Art
A Functionally Gradient Material (FGM) is an anisotropic composite material and can be a metal-ceramic material. A gradient in composition and/or microstructure which results in a gradient in composite properties is deliberately introduced into the material. Depending upon the application of the FGM, the gradient can be created in a continuous or stepwise fashion. For example, FGM's are used in aerospace applications where a graded interlayer between a relatively low coefficient of thermal expansion ceramic and a relatively high coefficient of thermal expansion metal relieves thermal stress by comparison with that created when the metal and ceramic are directly bonded. Thus, crack formation is prevented.
Typically, FGM's are prepared by (1) Chemical or Physical Vapor Deposition (CVD/PVD), conventional powder metallurgy processing, plasma spraying or Self Propagating High Temperature Synthesis (SHS). Powder metallurgy processing is the technique most commonly used for the preparation of composites from layers or plies having millimeter dimensions.
Conventional powder metallurgy processing involves consolidation processes that typically require binders which must be removed during some point of the process. These binders include thermoplastic organic binders which are removed by heating of the green composite before sintering of the molded part. Thermolysis of the thermoplastic binder generates volatile byproducts which must be removed from the part. Removal of such volatile byproducts is costly and must be accomplished in a carefully controlled manner if cracking, deformation or bloating of the part is to be avoided. Also, shrinkage control of individual layers is an important issue, especially when top and bottom layers have large differences in density. Conventionally, this is done by blending powders of various particle sizes as described by Takemura et al., "Evaluation of Thermal and Mechanical Properties of Functionally Gradient Materials of ZrO.sub.2 -Ni System", Ceramic Transactions, 1993, 34, 271.
Preceramic polymers are generally inorganic and organometallic polymers pyrolizable to yield one or more ceramic phases as a residue of the pyrolysis. It has been demonstrated, as described by Semen et al., "A Preceramic Polymer Route to Molded SiC Ceramic Parts", Ceram. Eng. Sci. Proc. 1991, 12, 1967, that preceramic polymers can be used successfully as binders in fabrication of shaped ceramic parts. Little evolution of gaseous byproducts is observed and parts having strengths of over 650 MPa have been fabricated. The use of preceramic polymers as binders for preparation of metal matrix composites has been reported in Yajima et al., "Heat-Resistant Fe-Cr Alloy with Polycarbosilane as Binder", Nature, 1976, 264, 237, in Japanese Patent No. 04-128338 and in Seyferth et al., "Application of Preceramic Polymers in Powder Metallurgy: Their Use as Low-Loss Binders and for in Situ Formation of Dispersed Ceramic Phases in the Metal Matrix", Chem. Mater., 1994, 6, 10.
Thus, there exists a need for a method of in-situ powder metallurgy processing of a functionally gradient material which avoids the use of those types of binders, which, when removed by thermolysis, generate volatile byproducts whose removal is relatively costly and often results in degradation of the properties of the material being formed. This method should also provide a means for controlling shrinkage of functionally gradient material during processing.